FrogReaver
The most respectful and polite poster ever
I was just quickly thinking of examples. Pick lock, possibly not help-able (although two players of rogues with expertise in thieves tools might well disagree!) But many other tasks, likely yes. Of course this will always be up to a DM - one could just squash helping.
Ah, so to get a "nearly impossible" result in tier-2 with +10 and advantage, is about 9% likely*. To get a "nearly impossible" result in tier-2 with +10, guidance and advantage, is about 41% likely. The cantrip is making quite a difference. Even without help, the change in getting a "nearly impossible" result is from 5% to about 18%.
However, it's also important to respect ability spiking via stacked buffs (which led 3rd ed to have that whole buff-types thing, and systems like Earthdawn to apply a rule of three). Guidance, being nigh-costless, is egregious in that it freely pushes everything upward. Above I think where the designers - from the extant text - appeared to have expected the limits to be.
Thinking through guidance, the only times it's really useful is if the skill in question can't be helped with or if another PC helps while another casts guidance.
For the first type of situation it's just replaced the help action so no big issue.
For the 2nd type of situation - those circumstances typically will involve a scene where spending your time casting guidance will mean your not able to do some other useful action. I don't see an issue with this.
For social encounters - there's already more bad with casting spells than benefit a +1d4 would grant
For actions the whole group must take - there's extremely limited benefit in giving a single member +1d4
Essentially, when actually examined, guidance is no where near a no cost +1d4 to all skills. That it can stack with the help action on a few checks is no issue - especially lock picking checks - I mean it's not like the PC's having a better chance at getting a locked door or chest opened that they had a chance to get opened to begin with is a bad thing - especially since the rogue is the only one likely to be able to pick locks anyways...